A typical turbofan gas turbine engine for powering an aircraft in flight includes a nacelle having an inlet providing ambient air to a fan. The fan is powered by a conventional power turbine disposed downstream from a core engine thereof.
During takeoff of the aircraft, the rotational speed of the fan reaches relatively high values, with the tips thereof obtaining transonic or supersonic speeds which generate noise. A typical component of the noise occurs at relatively low frequencies below about 1,000 Hertz, with multiple pure tone (MPT) noise radiated from the engine inlet being particularly annoying to passengers in the aircraft and to people on the ground.
The MPT noise can be suppressed or attenuated using conventional passive acoustic liners. A typical passive acoustic liner includes a perforated facesheet which lines the nacelle duct, and one or more surrounding honeycomb structures, with a rigid backplate therearound. The acoustic liners must be configured or tuned to specific frequency ranges in order to effectively attenuate noise. A typical nacelle duct includes two types of acoustic liners: a forward, relatively thin liner for attenuating relatively high frequency noise greater than 1,000 Hz; and an aft liner adjoining the forward liner which is relatively thick for attenuating the low frequency MPTs below 1,000 Hz. These liners are specifically configured for effecting a suitable impedance at the wall or facesheets thereof for passively absorbing or attenuating specific frequency noise. Acoustic impedance is a conventional term representing the ratio of the local acoustic pressure to the normal component of acoustic velocity at the facesheet and accounts for phase as well as magnitude.
Conventional analytical procedures are available for predicting optimum acoustic impedance for maximizing noise attenuation at given frequencies. However, the low frequency MPTs require relatively thick acoustic liners which therefore add undesirable space requirements as well as increase overall engine weight. The MPT acoustic liners either add undesirable extra length to the nacelle or reduce the length available for the high frequency acoustic liners. And, since the thickness of the MPT acoustic liner is proportional to the wave length of a relatively narrow bandwidth of noise, differently configured MPT acoustic liners would be required for a given engine for attenuating wider bandwidth noise.
Active noise control systems are known for injecting acoustic energy with suitable power to cancel pressure waves being generated by the noise source. These systems are also referred to as anti-noise systems since they generate opposite phase noise for canceling the source noise. However, these systems are relatively complex and require microphones at the listening location, and loud speakers for generating the anti-noise, and vary in effectiveness.